1. Field of the Invention
The present invention pertains to methods, procedures and apparatus for determining (i.e., diagnosing) the acceptability of an illuminating device, such as a fluorescent lamp that is used with an image scanner.
2. Discussion of Background and Relevant Information
Imaging devices, such as, for example, optical image scanners, employ an illuminating device (radiation emitting device), such as an elongated fluorescent lamp, to illuminate an object that is to be scanned. One of the problems faced by such an imaging device is that variations in the brightness or intensity of the lamp from one scanner to another scanner, from one scan to another scan, or from various portions of a single scan with any particular scanner, can occur. In order to obtain an accurate, uniform scanning, the amount of radiation (light) that is output by the fluorescent lamp must be uniform along its entire length. Any non-uniformity in the light source can translate into an inaccurate output scan of the object. For instance, new fluorescent lamps tend to be brighter at the center of the lamp as compared to their extremities. Alternatively, older lamps tend to be dimmer at their centers as compared to their extremities.
In addition, an output of a scanning array, such as, for example, a plurality of pixels in a charge coupled device (CCD) array employed in the imaging device, varies from pixel to pixel, due to, for instance, manufacturing tolerances. Accordingly, it is desirable to have a means for adjusting the apparent brightness of the lamp so as to obtain a uniform output from the imaging device.
Scanners have been designed that perform a "block pixel" calibration, wherein pixel-block-to-pixel-block variations are accommodated by adding or subtracting calculated block values from the block values outputted by the CCD array used in the scanner. Other scanners perform such a calibration on individual pixels, wherein pixel-to-pixel variations are accommodated within a particular range by adding or subtracting calculated values from the values obtained/output by the CCD array employed in the scanner.
The above-described calibrations are typically performed by initially positioning the CCD array proximate a uniform "white" strip, of a high reflectance material, just ahead of a leading edge of the object to be scanned. The brightness of the lamp and a "CCD sense window" of the scanner are set to a predefined default setting, and the calibration white strip is read. Based upon values obtained by the CCD, offset values are calculated which are used during a scanning operation in order to attempt to compensate for non-uniformity and to achieve a uniform output across the scanned image.
However, as noted above, the characteristics of the lamp are not consistent from one lamp to another lamp. For example, the amount of time required for the lamp to warm-up (referred to as a "warm-up" time) to produce a desired output level (brightness) varies from lamp to lamp. Hence, prior art scanners have no way for determining whether the lamp has warmed up sufficiently to begin the above-described calibration process.
Accordingly, prior art scanners have had to employ a fixed warm-up time period that is greater than that which is typically needed. After the warm-up time period elapses, the prior art scanners operate under the assumption that the lamp has reached the predetermined brightness level, and the calibration procedure described above is performed.
However, if the lamp reaches the predetermined brightness level in a time period that is less than the fixed warm-up time period, the scanning device must wait until the fixed warm-up time period elapses, thereby reducing the operating speed of the scanner. Similarly, the prior art scanners do not detect instances when the lamp has not yet reached the predetermined brightness level within the fixed warm-up time period. Thus, in that situation, the calibration process will begin with a non-stabilized operating light source, possibly affecting the quality of the scanning operation.
Furthermore, it has been observed that when the lamp nears the end of its useful life, the brightness of the lamp dramatically decreases. However, there is a point beyond which the image sensing device (CCD array) is no longer able to quantify the diminished level of light. This can occur with respect to the entire CCD array, or with respect to individual pixels of the array, leading to a scan that is not uniform.
Similarly, it has also been observed that a lamp can be overly bright, such that the image sensing device (CCD array) is no longer able to quantify the overly bright level of light. This can occur with respect to the entire CCD array, or with respect to individual pixels of the array. In such a situation, the increased brightness saturates the image sensing device, leading to a scan that is not uniform.